Cathode ray tube



Nov. 3o, 1343.l H BOERSCH 2,335,637

CATHODE RAY TUBE Filed Aug. 23, '1940 Inventor: 1 Hans Boersch,

His Attorney.

Eatented Nov. 30, 1943 CATIIIODE RAY TUBE Hans Boersch,Berlin-Charlottenburg, Germany,

assignor to General Electric Company, a corporation of New YorkApplication August 23, 1940. Serial No. 353,949

In Germany September 12, 1939 g claims. (c1. sis-1i) The presentinvention relates to a cathode ray tube useful for investigating objectsby electron optical means. I

In a cathode ray device such as an electron microscope in which a.visibleimage is formed. by the action of an electron stream, it oftenhappens that the image produced is of blurred character, thisblurringbeing due at least in part to the fact that the various components ofthe electron stream are of different velocities. Thus, in electronmicroscopes of the type in which the image is produced by projecting anelectron stream "through the .object under in. vestigation, electronswhich are slowed down in passing through the object fail to follow thesame paths as the electrons of higher velocity 'and consequently produceblurring of the image. 'A similar effect may be noted if the primaryelectron source is itself of such character as to give rise to electronsof varying velocities.

According to the present invention the dimculties described lin theforegoing are minimized by providing in proximity to theimagereproducing surface a braking electrode, preferably in the form ofa. foil or grating, for preventing slowly movingelectrons from reachingthe surface. It is found that in the use of the invention in an electronmicroscope the shadow of the grating which is produced on the imagesurface has little effect in obscuring the greatly magnified image ofthe object under investigation. v

The features which I desire to protect herein are pointed out withparticularity in the appended claims. The invention itself may best Vbeunderstood by reference to the following dev scriptiontaken inconnection with the drawing in which Fig. l shows schematically anelemen tary form of electron microscope embodying the invention andFigs. 2 and 3-respectively indicate modications of the invention.

Referring particularly to Fig. 1, there is shown anode. The object to beinvestigated is posiltioned as indicated at 'l in proximity to the anodeopening 5 and is arranged to be traversed by the electron stream. It isassumed that the object 'I is at least partially of electron-permeablecharacter and that it represents some ele- -`nientiigch as a minuteorganism, which is desired to be viewed asan enlarged reproduction ofthe same.

It will be understood that with the arrangement specified above, theelectron stream, afterv enlarged image or shadow of the object on asuitable reproducing surface; Such a surface, which may take the form ofa fluorescent screen or a. photographic film, is positioned in the pathy ffrom the electron stream all electrons which are of such low veIocityas to have a tendency to in dotted outline a sealed evacuated envelope li Asmall opening 5 provided in the end wall of the produce objectionableblurring of the image reproduced on the surface i0. Due to the fact thatthe braking electrode'is in close proximity to the f surface I0 theshadow which it casts on the surface does not obscure the 'object imageto an objectionable degree.

It is desirable to assure the presence of a reasonably uniform eldAbetween the electrode II and the extremity of the anode 3. YTofacilitate this result one may provide a-further grating,.

indicated at I2, as an electron-permeable diaphragm across the end ofthe anode 3. Thisr grating is maintained at the samepotential as theanode.

As an alternative means for improving the elements described inconnection with Fig. l*V

they are identically numbered except for the expedient of priming thevarious index numerals.

A still further variation which may prove useful in certain cases isshown in Fig. 3. Inthis case the braking electrode comprises a pair ofgratings I3 and I4 which are successively arranged and which aremaintained at a `common potential. The space between the gratings may beclosed up laterally'as by the provision of a circularly extendingconducting wall as indicated at l5. In the use of apair of gratings asindicated in Fig, 3 it is preferable to mount the gratings in suchfashion that their Wires cross A assaut? 2. A cathode ray tube forinvestigating objects by electron optical methods comprising means inanelectron-pervious electrode in the form of a perpendicularly so that.the projection of the two gratings represents a screen or grid..

The use of a braking electrode in the manner specified in the foregoingrequires certain precautions in order to prevent the accumulation ofcharges on the image-reproducing surface. Normally the reproducingsurface does not accumulate charges because secondary electronsliberated from it fly to the high voltage anode and thus help to`maintain the surface in discharged condition. I'he presence of thebraking electrode, however, tends to lessen this eil'ect so thatprovision should be made for imparting transverse conductivity to theimage reproducing surface as, for example, by giving it a metal coating.

The invention described herein may be used to advantage in investigatingthick objects which have previously been found impossible ofinvestigation by electron optical means because of the blurring effectsinitially referred to. Numerous additional modifications may obviouslybe made in the invention and I aim in the appended claims tov cover allsuch equivalent variations as come within the true spirit and scope ofthe foregoing disclosure. v

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. A cathode ray tube for investigating objects by electron opticalmethods comprising means including an electron lens system. forproducing an electron stream having a cross-sectional patternrepresentative of the particular object under investigation, a surfaceadapted to be affected by the electrons in the said stream to produce .avisible image corresponding to the cross-sectional pattern of thestream, an electron-pervious electrode lying in a surface having asubstantial component perpendicular to the direction of travel of theelectrons and interposed between the electron lens system and the saidsurface and in proximity to the latter, and means for applying a brakingpotential to the said electrode so as to prevent slow electrons in thestream from reaching the surface.

conductive grating interposed between the said electron lens system andthe said surface and in proximityto the latter; and means for applying abraking potential to the said electrode so as to prevent slow electronsin the stream from reaching the surface.

3. A cathode ray tube for investigating objects by electron opticalmethods comprising means for producing anelectron stream having acrosssec'tional pattern representative of the particular object underinvestigation, a generally tubular electrode arranged to be traversed bythe said stream for accelerating and directing the same. animage-reproducing surface adapted to be affected by the stream after itstraversal 'of the said electrode, and an electron-pervious brakingelectrode extending across the path of the electron stream in a regionbetween the 'said tubular electrode and the said image-reproducingsurface for preventing slow electrons in the stream from reaching thesurface, the said braking electrode being of curvilinear contour'lnplanes taken parallel to the path ofthe electron stream.

4. A cathode ray tube for .investigating objects by electron opticalmethods comprising means for producing an electron stream having across-sectional pattern corresponding to the particular object underinvestigation, a tubular electrode arranged to be traversed by thestream and serving to accelerate and direct th same, animage-reproducing surface adapted to be affected by the stream after itspassage through the said tubular electrode, an electron-pervious brakingelectrode interposed between the said tubular .electrode and theimage-reproducingA surface for preventing slow electrons. in the streamfrom reaching the said surface and electron pervious conductive means ofplanar character mounted across the end of the tubular electrode whichis adjacent to the braking electrode for producing a uniform elddistribution between such electrodes.

5. A cathode ray tube. according to claim 1 vcharacterized by the factthat the electron-pervi..

ous electrode comprises a pair of successively arranged conductivegratings which are maintained at a common potential.

. HANS` BOERSCH.

